Can I change the inventory costing method from, first in, first out to wheighted average cost?

Changing the Inventory valuation method is currently unavailable, Max.

In QuickBooks, we're unable to switch your inventory costing method from First In, First Out (FIFO) to Weighted Average Cost. Once you've selected a costing method, it’s set for your inventory items and can’t be changed later on. See this article for more information: Inventory valuation methods for cost accounting.

If you need to use a different costing method, consult with your accountant or bookkeeper about starting a new QuickBooks Online company file and setting the preferred Inventory valuation method from the beginning. Keep in mind, this means starting fresh with your inventory setup.

If you have any additional questions, please don't hesitate to reply below.

@Max-Alejandro-Flores-Sandoval 

You need to use a 3rd party inventory management app or switch to QB Desktop.